The present invention relates to fossil fuel combustion, and, in particular, to removal of carbon dioxide from fossil fuel combustion gas mixtures.
Carbon sequestration refers to the capture and storage of carbon that would otherwise be emitted to, or remain, in the atmosphere as carbon dioxide (CO2). Fossil fuels are the main source of world fuel and supply over 85% of all primary energy, with nuclear, hydroelectric, solar, wind, and geothermal making up the rest. Although significant efforts and capital investments have been made by many nations to increase renewable energy options and to foster conservation and efficiency improvements of fossil fuel utilization, climate change during the coming decades will likely require significant effort and resources be expended on carbon sequestration.
The potential impact of increasing concentrations of greenhouse gases in the atmosphere will be a global concern well into the 21st century and perhaps even beyond. There are three primary initiatives that the U.S. Department of Energy has undertaken to address the buildup of greenhouse gases. These include the improved efficiency of energy utilization, the adoption of low carbon fuels, and carbon sequestration. Carbon geosequestration is the process of not only capturing CO2 emissions that would otherwise be lost into the atmosphere, but permanently storing them in geologic formations such as oil and gas reservoirs, abandoned coal seams, and deep ocean waters.
The primary sources of CO2 are fossil-fueled power plants, industrial processes, and in the future, by-products of fuel decarbonization plants. Power plants are the largest source and emit more than one-third of all CO2 emissions worldwide. There are three primary routes for capturing power plant CO2 effluent: flue gas separation, oxy-fuel combustion, and pre-combustion separation. While all three approaches have been piloted, flue gas separation is currently the most common commercial method.
Flue gas separation and CO2 capture are based on chemical absorption whereby CO2 is reacted with an amine and captured as an adduct in the liquid phase. The most commonly used absorbent for CO2 is monoethanolamine (MEA), which carries with it significant problems such as decomposition products requiring hazardous waste disposal and high corrosivity to processing equipment.
Attempts have been made to address the problems associated with MEA treatments. For example, U.S. Pat. No. 4,971,718 discloses the addition of antimony and N-methyldiethanolamine (MDEA) to an aqueous solution of MEA. The combination of antimony and MDEA was supposed to inhibit corrosion and retard solvent degradation. However, antimony compounds present potentially serious toxicity issues, and their use in large industrial applications could be problematic.
U.S. Pat. No. 4,477,419 discloses the use of copper salts in an alkanolamine solution in conjunction with the use of an activated carbon or ion exchange resin. The copper salts combined with the use of an activated carbon or ion exchange resin are supposed to lower corrosivity and/or degradative quality of the solvent. However, the use of carbon beds or iron exchange resins adds further cost and complexity to the CO2 capture process.
In U.S. Pat. No. 4,596,849, a thiourea-amine-formaldehyde based polymer is identified as a corrosion inhibitor for CO2 scrubbing using aqueous MEA. However, the process of U.S. Pat. No. 4,596,849 may produce carcinogens. In particular, the thiourea-amine-formaldehyde based polymer upon deterioration can release some level of formaldehyde, a suspected human carcinogen.
While research has been conducted using amines and amine derivatives as absorption aids, there remains an opportunity for an innovative technology that displays improvement in reduction of corrosion in MEA solutions, an increase in stability of the solvent, and/or less corrosion to the metal processing equipment found in towers and tanks.
It is, therefore, an object of the present invention to provide aqueous alkanolamine-based carbon dioxide absorbent solutions which exhibit low corrosivity to metallic equipment and do not contain or produce compounds which are toxic or harmful to the environment.